There are known integrated systems of sensors for the monitoring of key variables in the dynamics of the vehicle with use of complex technologies both in the sensor and data transmission field.
One of the greatest difficulties in equipping the tyre with sensors used in the prior art, lies in the data transmission and supply system. Conventional devices, such as frictional contacts and batteries, have a limited duration, low reliability, significant dimensions and introduce noise into the measuring system, and cannot thus be used in operating conditions. Many studies have therefore focused on transmitting the data by means of wireless systems, on the use of passive sensors and on on-site energy harvesting devices with significant devices for applications of an industrial nature. In addition, the installed sensors must be small and economical since the tyres are motor industry components having a relatively low cost. As regards indirect monitoring, the variable of interest is extrapolated from the magnitudes acquired by the sensors, generally the speed of the vehicle and the angular speed of the wheel. Yi et al [IEEE T. Contr. Sys. T. 2002, 10, 381-392] used wheel spin, the speed of the vehicle and the wheel load to determine the coefficient of friction. Since the phenomena that govern tyre dynamics are non-linear, it is difficult to establish general and reliable analytical reactions between measured magnitudes and magnitudes to be identified. In some cases, techniques based on fuzzy logic (Zhang, X. et al, In Proc. of IEEE Intelligent Vehicles Symposium, 2005; pp. 875-881) or on Kalman filters (Gustafsson, F. et al, SAE Tech. Papers 2001, 2001-01-0796) can be relied upon. Albeit indirect monitoring of the variables is easily achieved, since it takes advantage of existing sensors, this process has a modest precision and requires a new calibration each time the tyre is inflated or replaced.
The techniques based on the direct detection of the variables have better accuracy. Since sensitivity is linked to the size of the sensor, MEMS/NEMS devices lend themselves very well for producing accurate pressure measurements, even with a good spatial resolution (Nabipoor, M. et al, J. Phys.: Conf. Ser. 2006, 34, 770-775). Tjiu et al (Tjiu, W. et al, In Proc. of IEEE International Conference on semiconductor Electronics, 2004; pp. 350-353) used a MEMS device to monitor the operating conditions of the tyre. Yi (Yi, J., IEEE-ASME T. Mech. 2008, 13, 95-103) used a polyvinylidene fluoride-based to measure tread deformation. Devices that use surface acoustic waves (SAW) have also been used to monitor tyre deformation (Pohl, A. et al, IEEE T. Instrum. Meas. 1999, 48, 1041-1046),
Since all these sensors primarily comprise of highly rigid materials, an increase in sensitivity is only possibly by inducing flexional states of deformation and achieving very thin thicknesses (Shin, K. et al, Sens. Actuat. A 2005, 123-124, 30-35). This causes in general a high wear of the devices and limits the use thereof.
Accelerometer sensors are the instrument mainly adopted to monitor instantaneous grip in tyre-road contact.
Processing of the acceleration signals detected on the internal surface of the tread allows certain information on the speed range in the contact zone between tyre and road surface to be obtained and allows, by means of suitable algorithms, identification of the tyre grip. Certain drawbacks characterise this technical solution. Firstly, current technologies provide, for the same sensor dimensions, the installation of few accelerometers, which scan a single segment of the contact area during rotation of the tyre. There are also demanding technical problems linked to the electrical supply difficulty of the sensor and to the radio transmission of the information outside the tyre. The power required for such supply can be produced inside the tyre itself by means of electromechanical systems that take advantage of the relative motion of small floating masses and inductive effects, or by means of other systems that anyhow require installation within the tyre carcass of an electrical power generator device. In addition, the information associated with the electric signal generated by said sensors needs to be sent outside the tyre, thus requiring a radio transmission system for the information, which must also be necessarily housed inside the tyre.
Disadvantageously, both the direct or indirect measurement methods permit the acquisition, with low spatial-temporal resolution, of the variables, and are not therefore capable of monitoring and transmitting the dynamic variables following an increase in speed. Further studies and technological developments are therefore necessary in order to increase said resolution.
With regard to wireless transmission systems, data can be transferred from the tyre to the receiver through active or passive devices. Some simple systems use the resonance of a capacitive-inductive unit and require to be powered in order to send the radio signal (Yi, J., IEEE-ASME T. Mech. 2008, 13, 95-103). The main limitations are the limited duration and the dependence on the temperature of the power supply batteries. This restricts their use to laboratory tests. For use in operating conditions, it is therefore necessary to adopt passive or onsite power supply devices through energy harvesting techniques.
Energy harvesting consists of converting mechanical energy into electrical energy through capacitor generators (Meninger, S. et al, IEEE T. Vlsi. Syst. 2001, 9, 64-76), electromagnetic or piezoelectric generators (Jeong, S. et al, Sens. Actuat. A 2008, 148, 158-167). However, to date, the electrical energy provided is low, less than 1 mW/cm2, and is insufficient to adequately acquire and transmit the signal.
A typical passive device is based on the electromagnetic coupling of two inductors (Jachowicz, R. et al, Sens. Actuat. A 2000, 85, 402-408). Matsuzaki et al (Matsuzaki, R. et al, Adv. Compos. Mater. 2005, 14, 147-164) produced such a device through a resonant circuit formed by an inductor and a capacitor. By renouncing the magnetic coupling, Schimetta et al (Schimetta, G. et al, IEEE T. Microw. Theory 2000, 48, 2730-2735) produced a SAW transponder to supply a capacitive pressure sensor. However, for applications in operating conditions it is still necessary to improve the compatibility between the passive sensor and the tyre.
Disadvantageously, all the sensor productions of the prior art, applied for measuring the grip between tyre and road surface, present considerable wear phenomena and low reliability against the high cost and complexity of the entire system comprising the sensor, the respective supply and data transmission system.
There is therefore a need to produce a system for measuring the grip between tyre and road surface, which allows the aforementioned drawbacks to be overcome.